Storage screen for direct-viewing storage tube



Nov. 18, 1958 a. F. SMITH EI'AL 2,861,207

STORAGE scasrsn FOR DIRECT-VIEWING STORAGE was Filed April 1, 1957 2 Sheets-Sheet 1 George F. Smith,

42 Wmmmmm Henry M. Smith F/g 3 40 5m.

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Nov. 18, 1958 s. F. SMITH ETAL 2,861,207

s'romcs scnssn FOR DIRECT-VIEWING STORAGE TUBE Filed April 1, 1957 2 Sheets-Sheet 2 George F. Smith, Henry M.

INVENTOR B). F lg. 6. ah (1 m.

AGE/VI United States Patent STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE George F. Smith, Los Angeles, and Henry M. Smith, Pacific Palisades, Calif., assignors to Hughes Aircraft Company, Culver City, Calif a corporation of Delaware Application April 1, 1957, Serial No. 649,721

6 Claims. (Cl. 313-68) This invention relates to direct-viewing storage tubes of the mono-tone type and particularly to a storage screen for use in this type of tube that has a storage surface on which substantially higher writing speeds may be eifected.

In the operation of a direct-viewing storage tube, an electron beam of elemental cross-sectional area is modulated with an image signal and scanned over a storage screen to produce a corresponding charge replica. The resulting charge replica thus produced is acted upon by flood electrons to decrease all charges less than a critical potential to the potential of the associated flood gun cathode and to charge all potentials greater than the critical potential to the potential at which the collector grid is maintained. Accordingly, the potentials at which the flood gun cathode and the collector grid are maintained are referred to as stable potentials. The resulting mono-tone charge replica thus produced is, in turn, employed to control the flow of other flood electrons to a viewing screen to produce a visual presentation of the charge replica. It is thus apparent that the scope of the applications of such a direct-viewing storage tube is dependent on the rapidity with which charge replicas can be produced, i. e., on the writing speed capable of being attained andthe length of time that the charge pattern can be retained on the storage screen. Both of these parameters are functions of the characteristics of the storage screen employed in the present direct-viewing storage tube.

In order to permit stable storage, i. e., to retain a charge pattern wherein the charged areas do not shrink or grow, it is necessary that the storage surface be provided by a dielectric material that is granular in form, e. g., a powder. If the surface were smooth such as the type prepared by vacuum evaporation, there would be no stable range at all. The reason for this phenomenon is that the flood or viewing beam of electrons produce what is known as bombardment induced conductivity which conductivity tends to discharge the sharp boundary between written and unwritten areas and cause the boundary therebetween to move one way or the other so that the image produced by the stored pattern turns all white or all black. The use of a powder dielectric serves to break up the surface sufiiciently so that'there is a range of operating potentials such that no boundary movement takes place. Thus, it is evident that a storage screen composed of a powder dielectric is requisite for proper stable operation of a direct-viewing storage tube in the aforementioned manner.

From the above, one realizes that bombardment induced conductivity produced by flood electrons is detrimental to the operation of a direct-viewing storage tube unless minimized by breaking up the storage surface into granules. On the other hand, the writing of a charge pattern on the storage surface can be shown to be a simple process of charging a capacitor, i. e., the storage dielectric, by means of secondary electron emission produced by a high energy electron beam of elemental Patented Nov. 18, 1958 ice cross-sectional area. Since this writing takes place in a very short interval of time, the bombardment conductivity efiects are relatively unimportant during this phase of operation even though the higher energy beam might be expected to produce more such bombardment conductivity. In particular, the writing speed depends upon the inherent capacitance of the storage dielectric, upon the voltage required to be written and upon the secondary electron emission ratio of the storage surface. Hence, it is desirable that the dielectric material which provides the storage surface be reasonably thick to decrease the storage capacitance to the extent that the collector capacitance predominates and that the surface of the dielectric material provide good secondary electron emission.

According to the above, the requirements for making an eflicient storage screen for use in the aforementioned type of tube are, first, the dielectric material for providing the storage surface must be a powder; it must be capable of being sprayed; and to effect a stable range, there are certain powder parameters to be optimized. The powder particle size, for example, should be relatively large and uniform to make the bombardment conductivity produced by the flood beam comparatively low thereby effecting a large stable range. Secondly, the secondary electron emission from the storage surface should be as high as possible for high speed writing.

It is therefore the object of the present invention to separate the above two requirements from their overly restrictive dependence upon one another and thus provide a storage screen with a storage surface of overall optimum characteristics. 7

Another object of the invention is to provide a storage screen having a granular storage surface with low bombardment induced conductivity, particularly from granule to granule, and high secondary electron emission.

Still another object of the present invention is to provide a process for enhancing the secondary electron emissive characteristics of a storage screen particularly adapted to store or retain mono-tone charge images.

In accordance with the present invention, a directviewing storage tube incorporating a storage screen with a storage surface having the above-mentioned properties is attained by first assembling a storage screen in the manner described in United States Patent No. 2,788,467, entitled Direct-Viewing Storage Tube, issued April 9, 1957, by Henry M. Smith. In this respect, talc has been found to be an optimum dielectric material for providing the storage surface because it sprays well and simultaneously has very high secondary electron emission. Magnesium fluoride, on the other hand, has a greater secondary electron emission ratio, but can hardly be sprayed at all and will not build up to the required thicknesses because of its physical properties. in powder form. In accordance with the present invention, a final thin evaporated layer of dielectric material with a high secondary electron emission ratio such as, for example, magnesium fluoride is then applied over the entire area of the storage screen. This final secondary electron emitting layer can even be applied over the contactingcollector grid of the completed storage screen as a last step before sealing into the tube envelope to maintain easy freedom from contamination. Thus, in practice, the evaporation may be done over the contacting aluminum collector grid if this type of storage grid assembly is used as no harmful electrical effects result since the layer of magnesium fluoride is sufiiciently thin as to prevent any appreciable voltage from being built up across it and thus interfere with the function of the collector grid.

This final evaporated layer of, for example, magnesium fluoride should have the highest possible secondary electron emission and preferably should show low bombardment induced conductivity, particularly from grain to grain. In order to accomplish the intended purposes of the present invention, there are two limits to the thickness of the evaporated layer of magnesium fluor de employed. On the one hand, the layer must be thick enough to give its characteristic secondary electron emission to the storage surface and, on the other hand, it must not be so thick as to produce a continuous smooth film over the entire target and thus result in a complete loss of astable range of operation. It has been found that this range of thickness is from 0.2 to 2 or 3 wavelengths of mercury green light (5461 Angstroms) or sodium yellow light (5890 Angstroms). Thus, in actual practice this range of thickness is from 800 Angstroms to from 10,000 to 20,000 Angstroms.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 illustrates a cross-sectional schematic presentation of a direct-viewing storage tube incorporating the storage screen of the present invention;

Fig. 2 shows a plan 'view of an enlarged portion of the storage screen of the storage tube of Fig. 1;

Fig. 3 shows section 3-3 of the plan view of the enlarged portion of the storage screen shown in Fig. 2;

Figs. 4 and 5 illustrate greatly enlarged cross-sectional views of the granular characteristic of the storage surface with correct and incorrect thicknesses, respectively, of the secondary electron emissive layer thereon; and

Fig. 6 is a schematic drawing illustrating an example process for applying the secondary electron emissive coating of the present invention to the storage screen of the storage tube of Fig. 1.

Referring to Fig. 1, there is shown a direct-viewing mono-tone storage tube of the type which is more fully described in United States Patent No. 2,788,466, entitled Direct-Viewing Storage Tube, issued to Siegfried Hansen on April 9, 1957, and assigned to the assignee of the present application. For a better understanding of the invention, an embodiment of the storage screen of the invention is described that is particularly adapted for operation in the type of direct-viewing storage tube disclosed in the above Patent No. 2,788,466 issued to Siegfried Hansen. This type of direct-viewing storage tube comprises an evacuated bulbous envelope l0 having a neck portion 11 and a flat end portion 12 at opposite extremities. The neck portion 11 of envelope houses an electron gun 14 which is coupled to a signal source 16 for producing an electron beam that is modulated in accordance with the signal. This modulated electron beam is focused by means of a focusing coil 18 and deflected by means of pairs of deflecting coils 20, 22.

Commencing from the inner surface of the flat end portion 12 of envelope 10 and proceeding; towards the gun 14, a viewing screen 24, a storage screen 26 of the present invention, and an ion repeller grid 28 are disposed in the order named. Last, a flood gun 30 is disposed about the path of the electron beam adjacent the neck portion 11 which, in conjunction with electrodes 32, 33 and 34 disposed concentrically about the inner surface of envelope 10 intermediate gun 14 and the end portion 12, is adapted to direct flood electrons uniformly over the entire area of the storage screen 26. In operation, the modulated electron beam is scanned over the storage screen 26 by means of the deflecting coils 20, 22 to produce a charge replica of the signal on its storage surface. This charge replica is then acted upon by the flood electrons whereby it is converted into a mono-tone charge replica which controls the flow of other flood electrons from the flood gun 30 to the viewing screen 24 to produce a visual presentation of the charge replica.

Referring to Figs. 2 and 3, there is shown plan and sectional views, respectively, of an enlarged portion of the storage screen 26 of the direct-viewing storage tube illustrated in Fig. l. The storage screen 26 comprises a support mesh 40 on one side of which is disposed a uniform layer 42 composed of granules of dielectric material such as, for example, ball-milled talc and a thin aluminum collector grid 44 disposed in contact with the layer 42 of dielectric material whereby the areas of layer 42 within the interstices of grid 44 provide storage surface. The thickness of layer 42 is of the order of from 1 to 2 mils. Further, the meshes of the aluminum collector grid 44 are disposed at an angle with those of support mesh 40 so as to minimize what is known as moire effect on the flood electrons flowing to the viewing screen 24. The above-mentioned storage screen 26 may be manufactured in accordance with the teachings of the aforementioned Henry M. Smith Patent No. 2,788,467.

In accordance with the present invention and referring particularly to Fig. 4, a final thin evaporated film 46 of dielectric material having high secondary electron emissive characteristics such as, for example, magnesium fluoride is applied over the entire area of the storage screen 26. This evaporated film 46 may be applied before or after the aluminum collector grid 44 is disposed in contact with the storage surface. It is preferable, however, to apply the evaporated film 46 iust prior to inserting the storage screen 26 within the tube envelope 10 so as to minimize any possibility of contamination of the storage surface. The evaporation of the high secondary electron emissive dielectric over the metallic parts of the storage screen does not cause any detrimental electrical effects since the film 46 is sufliciently thin as to prevent appreciable voltage build up across it.

Upon disposing the film 46 of magnesium fluoride over the storage screen 26, there is a certain range of thickness necessary to impart the aforementioned desired characteristics to the storage surface. First, in order for magnesium fluoride to impart its secondary electron emissive characteristics to the storage surface, a certain mini mum thickness of film 46 is necessary. A conservative lower limit of this minimum thickness is calculated to be the thickness of magnesium fluoride that can be penetrated by a 2 kilovolt electron beam. Actually, since secondary electrons can escape from the entire penetration thickness as well as from nearer the surface, it is evident that somewhat thinner films may be almost as effective as that calculated above. With regard to the calculation of the thickness of the film 46 composed of magnesium fluoride, it can be shown that the penetration power of an electron beam through aluminum is nearly the same as that through magnesium fluoride. It has been shown that a 3 kilovolt electron beam can penetrate a layer of aluminum 800 Angstroms thick. A thickness of 800 Angstroms corresponds to 0.2 wavelength of either mercury green or sodium yellow light. Referring to Fig. 4, there is shown a highly magnified section of the conductive support mesh 40 and the layer 42 composed of granules of dielectric material on which is disposed film 46 of magnesium fluoride which film 46 is of the correct relative thickness in accordance with the present invention. It is to be noted that the film 46 of magnesium fluoride does not form a continuous layer over the individual granules of dielectric material constituting layer 42.

On the other hand, a film of magnesium fluoride over layer 42 of talc will become a continuous smooth film when thicknesses of the order of from 2 to 3 wavelengths of either mercury green or sodium yellow light are reached. This range corresponds to the thicknesses of the order of from 10,000 to 20,000 Angstroms. It is to be noted that even a thickness of 20,000 Angstroms is only 0.00008 inch whereas the thickness of layer 42 is generally of the order of 0.002 inch. In accordance with the in this figure, it will be too thick to achieve the characteristics desired for the storage screen 26 of the present invention. The maximum range of thickness may be determined by analogy to the characteristics of a film of aluminum evaporated on talc. That is, a film of aluminum having a thickness of from 2 to 3 wavelengths of mercury green or sodium yellow light begins to conduct at this thickness and hence may be considered as being on the verge of becoming continuous.

As is evident from the above, it is necessary to control the thickness of the film 46 of magnesium fluoride evaporated over the storage surface of the storage screen 26. Methods of controlling the thicknesses of magnesium fluoride evaporated over lenses are quite well known in the art. Hence, a similar process may be adapted in manufacturing the storage screen 26 in accordance with the present invention. An example of such a process is illustrated in Fig. 6 wherein the storage screen 26 is disposed within an evacuated chamber 52 above a boat 54 filled with magnesium fluoride to be evaporated. A lens 56 is mounted at an adjustable distance from the boat 54 containing the magnesium fluoride and is adapted to be illuminated by a sodium yellow lamp 58 in the manner shown in the drawing.

If the lens 56 is at same distance from the boat 54 as is the storage screen 26, it will indicate a thickness of one-quarter wavelength at the first minimum of reflection. By moving lens 56 closer or farther from boat 54, one can get less or more on the storage screen 26 than at the A wavelength point of the lens. A lens is preferable to a piece of plane glass for this purpose, since the images of the lamp 52 from the two sides thereof can be separated by the eye. Furthermore, the index of refraction of magnesium fluoride is better suited for giving minimum reflection from a high index (optical) glass of the type used for lenses.

Thus, to evaporate a film 46 of proper thickness on the storage screen 26, a current is caused to flow through the boat 54 to evaporate the magnesium fluoride until the reflected image of the lamp 58 from the side of the lens 56 nearest boat 54 goes to extinction for the first time. The second minimum reflection point corresponds to A wave length thickness. For sodium yellow light these thicknesses would correspond to 1470 and 4400 Angstrom units, respectively, the desired range being from 800 to 10,000 Angstroms.

What is claimed is:

1. In an electronic storage tube, a target element comprising a conductive'member for providing a support surface, a comparatively thick layer composed of individual granules of dielectric material disposed over at least a portion of said support surface to provide storage surface, and a film of magnesium fluoride disposed over said storage surface.

2. In an electronic storage tube, the target element as defined in claim 1 wherein said film of high secondary electron emissive dielectric material is constituted of mag nesium fluoride and is from 800 to 20,000 Angstroms thick.

3. In an electronic storage tube, a target element comprising a conductive member for providing a support surface, a uniformly thick layer of individual granules of talc disposed over at least a portion of said support surface thereby to provide storage surface, and a film of magnesium fluoride disposed over said storage surface, the thickness of said film being from 800 to 20,000 Angstroms.

4. In an electronic storage tube the target element as defined in claim 3 wherein said uniformly thick layer of individual granules of tale is from 0.001 to 0.002 inch thick.

5. In a direct-viewing storage tube, a storage grid assembly comprising a first metallic screen, a comparatively thick layer composed of granules of dielectric material disposed over one side of said first metallic screen coextensive with the meshes thereof, a second metallic screen disposed in contact with said comparatively thick layer composed of granules of dielectric material whereby the areas of said layer within the interstices of said second screen provide storage surface and a thin evaporated film of high secondary electron emissive dielectric material disposed over said second screen and said storage surface.

6. A direct-viewing mono-tone storage tube including a storage grid assembly comprising an electroformed nickel screen of predetermined thickness; a layer composed of ball-milled talc disposed over one side of said nickel screen coextensive with the meshes thereof, the thickness of said layer being comparable to said predetermined thickness; an aluminum screen that is thin compared to said predetermined thicknessdispcsed in contact with said layer whereby the areas thereof within the interstices of said aluminum screen provide storage surface; and a thin evaporated film of magnesium fluoride disposed over said aluminum screen and said storage surface, the thickness of said film being from 800 to 20,000 Angstroms.

Referenm Cited in the file of this patent UNITED STATES PATENTS 8 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 4 November 18, 1958 Patent No. 2,861,207

George F. Smith et al.

It is herebfif certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 8, after "film", strike out "of high secondary"; line 9, strike out "electron emissive dielectric material is constituted"; -line 10, after "fluoride" strike out and Signed and sealed this 10th day of March 1959. I

(SEAL) Attest:

KARLH. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

